The present invention relates to a mechanically controllable power booster for actuation of a hydraulic master cylinder, in particular, for hydraulic brake systems of automotive vehicles, comprising a booster housing fastened to the open end of the housing of the master cylinder, a booster piston movable in the booster housing and subdividing the housing's interior into two chambers, namely a low pressure chamber and a working chamber, a valve device to control the pressure prevailing in the working chamber carried by the working piston and a reaction device which adds, upon actuation, the force of an actuating rod connected with the control element of the valve device to the force delivered by the booster piston in a predetermined ratio and transmitting the combined forces to a push rod actuating the master cylinder.
In a known power booster of this type, for example, German Pat. No. DE-OS 2,922,299, the actuating rod projects from the booster housing on the side thereof remote from the master cylinder and is pressed into the booster housing for actuation of the power booster. The booster housing is fastened to a mounting plate of the vehicle on the side of the actuating rod. For the purpose of relieving the load from the booster housing and reducing the weight of the power booster, there is provided a slotted tube axially penetrating the interior of the booster housing, which tube transmits the forces occurring between master cylinder and the housing's attachment at the vehicle. This known power booster entails considerable effort and cost and its effective working surface is reduced by the annular cross section of the tube.
There is another known power booster for a hydraulic brake system in automotive vehicles, for example, German Pat. No. DE-PS 543,640, in which the booster housing is secured to an open end of the master cylinder and the booster piston is via a push rod rigidly connected with a first piston of the master cylinder. The actuating rod projects from the bottom of the master cylinder remote from the booster and is connected to a second piston which is spaced from the first piston of the master cylinder. Located between the end of the actuating rod and the brake pedal is the valve device which communicates with a working chamber of the power booster via a hose line extending parallel to the master cylinder. A reaction device is not employed in this known power booster. A disadvantage of this known power booster is that its control is purely travel-responsive, permitting the driver to "feel" the magnitude of the brake force actually generated only indirectly through the vehicle deceleration and, as a result, there is the danger of over-braking the vehicle. Another disadvantage is that the actuating force applied at the brake pedal is not able to be utilized for the generation of a working pressure in the master cylinder in the case of an inoperable power booster.
In still another known power booster for hydraulic brake systems of automotive vehicles, for example, U.S. Pat. No. 2,900,962, the booster housing is arranged on the end of the master cylinder and the booster piston is connected to the piston of the master cylinder via a tube. Inside the tube, the actuating rod is arranged. The actuating rod penetrates the piston of the master cylinder and the tube and bears against the booster piston through a reaction device formed by levers. In this known power booster, the booster housing is loaded by solely those pressures which prevail in its inside. The pressure forces introduced at the actuating rod are absorbed directly by the housing of the master cylinder and supported at the housing's fastening to the vehicle. This known power booster has the disadvantage of being expensive to manufacture and of requiring the diameter of the master cylinder to be increased due to the arrangement of the actuating rod inside the tube so that part of the weight reduction which may be achieved by this structure will be consumed by the higher weight of the master cylinder.